The goal of this STTR is to identify novel small molecule compounds targeting the nitrogen respiration pathway of Pseudomonas aeruginosa. This pathway is crucial to the pathogenesis of P. aeruginosa in vivo, but has not yet been exploited for antimicrobial drug discovery. There is a clear need for new treatments to combat P. aeruginosa infections. Antibiotic resistance is exhibited by most clinical isolates and has a marked, negative impact on our ability to treat and cure such infections in patients. In addition, hospital acquired multi-drug resistant Gram-negative infections are estimated to increase per-patient costs >$46,000. Our preliminary results show that genetic or chemical inhibition of nitrogen respiration in P. aeruginosa prevents anaerobic growth, limits biofilm formation, and decreases fitness of P. aeruginosa in murine models of pulmonary infection, indicating the potential of small molecule inhibitors of nitrogen respiration as strong candidates for a new type of antimicrobial agent. In addition, our preliminary analysis of the crystal structure of the Escherichia coli nitrate reductase NAR, which has high sequence homology to the not-yet-crystallized P. aeruginosa enzyme, suggests opportunities for small molecule binding and inhibition. We have developed a whole organism high-throughput screening assay that measures inhibition of nitrogen respiration. We plan to screen 250,000 compounds from 11 libraries at the ICCB facility at Harvard Medical School in this Phase I project. Control assays will be performed to exclude false positives. Our team of chemists will analyze data from the screen to identify and prioritize hits with drug development potential. Hits will be purchased and/or resynthesized for follow-up studies to confirm inhibition of anaerobic growth and positive hits will then be screened for inhibition of biofilm formation. Compounds will also be screened for cytotoxicity in human cell lines. At the end of this Phase I project we expect to have 5 or 6 lead compound series to move forward into further drug development (i.e. increasing potency, selectivity and drug-like properties) in a Phase II proposal.